Alliance forces had pushed at least 10 kilometres (six miles) south towards the city from the towns of Ain Issa and Suluk, she said.

In both cases SDF fighters were still some distance from Raqa — on the Ain Issa front at least 30 kilometres (20 miles) away.

“The offensive is going according to plan,” said Ahmed, who added that at least 10 villages had so far been taken from IS. “The battle will be long.”

– ‘Fight will not be easy’ –

An SDF commander said IS was fighting back with its favourite tactic of sending suicide bombers in explosives-packed vehicles against advancing forces.

“IS is sending car bombers but coalition planes and our anti-tank weapons are limiting their effectiveness,” the commander said, speaking on condition of anonymity.

The SDF says some 30,000 of its fighters are taking part in the operation, dubbed “Wrath of the Euphrates”. It aims to surround and isolate IS inside Raqa before eventually assaulting the city itself.

Both SDF commanders and coalition officials have warned that the battle is likely to be long and difficult.

“As in Mosul, the fight will not be easy and there is hard work ahead,” US Defence Secretary Ashton Carter said in a statement after the assault was launched.

“But it is necessary to end the fiction of ISIL’s caliphate and disrupt the group’s ability to carry out terror attacks against the United States, our allies and our partners,” Carter said, using an alternative name for IS.

Driving IS from both cities has been the endgame since the US-led coalition launched air strikes against it in summer 2014, shortly after the jihadists seized control of large parts of Syria and Iraq.

Some 50 US military advisers are involved in the Raqa operation, particularly to guide air strikes, according to an SDF source.

Near Ain Issa, an AFP correspondent saw at least one soldier who had US markings on his helmet with SDF fighters.

– Battles rage near Mosul –

Near Mosul, federal police, army and elite interior ministry forces established full control over Hamam al-Alil, the last town of note on the way to Mosul from the south, AFP reporters said.

It lies on the west bank of the Tigris river, about 15 kilometres (nine miles) southeast of the edge of Mosul.

Fighting also continued east of Mosul, with Kurdish forces launching an offensive to take the town of Bashiqa and the elite Counter Terrorism Service battling IS in the city’s suburbs.

“Up to seven neighbourhoods are under the control of counter-terrorism forces, and they are now completely securing them and clearing them of pockets of terrorists present inside the houses,” CTS spokesman Sabah al-Noman told AFP.

The Mosul offensive has advanced faster than expected but the battle for Raqa is far more complicated.

Unlike in Iraq where the coalition has a state-controlled ally in federal forces, in Syria its ground partner is made up of local militias, including some rebel groups that have battled President Bashar al-Assad’s regime.

The domination of the SDF by the Kurdish People’s Protection Units (YPG) has also raised deep concern with Turkey, which considers the YPG a “terrorist” group linked to the outlawed Kurdistan Workers’ Party (PKK).

Turkey in August began its own operation inside northern Syria, targeting both IS and the YPG and supporting local rebel forces.

Aid groups have voiced concerns for civilians trapped in both Mosul and Raqa, warning they may be used as human shields.

More than 34,000 people have been displaced since the Mosul operation began on October 17, the International Organization for Migration said on Monday.

More than a million people are believed to be in Mosul. Raqa had a population of some 240,000 before 2011 but more than 80,000 people have since fled there from other parts of Syria.